Semiconductor integrated circuit and method of fabricating same

ABSTRACT

A semiconductor integrated circuit comprising thin-film transistors in each of which the second wiring is prevented from breaking at steps. A silicon nitride film is formed on gate electrodes and on gate wiring extending from the gate electrodes. Substantially triangular regions are formed out of an insulator over side surfaces of the gate electrodes and of the gate wiring. The presence of these substantially triangular side walls make milder the steps at which the second wiring goes over the gate wiring. This suppresses breakage of the second wiring.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an integrated circuit comprising aninsulating substrate on which insulated-gate semiconductor devices(TFTs) in the form of thin films are formed and also to a method offabricating the integrated circuit. The insulated substrate referred toherein means a whole object having a dielectric surface and embracessemiconductors, metals, and other materials on which an insulator layeris formed, unless stated otherwise. Semiconductor integrated circuitsaccording to the invention can be used in various circuits and devices,such as active matrix circuits of liquid crystal displays, theirperipheral driver circuits, driver circuits for driving image sensors orthe like, SOI integrated circuits, and conventional semiconductorintegrated circuits (e.g., microprocessors, microcontrollers,microcomputers, and semiconductor memories, and so forth).

2. Description of the Related Art

Where an active matrix liquid crystal display, an image sensor circuit,or other circuit is formed on a glass substrate, use of integratedthin-film transistors (TFTs) has enjoyed wide acceptance. In this case,it is customary to first form a first wiring including a gate electrode.Then, an interlayer insulator layer is formed. Subsequently, a secondwiring is formed. If necessary, a third and even a fourth wirings may beformed.

A serious problem with such a TFT integrated circuit is that the secondwiring breaks at the intersections of this second wiring and a gatewiring which is an extension of a gate electrode. This is caused by thefact that it is difficult to form an interlayer insulator layer over agate electrode and wiring with a good step coverage and to flatten theinsulator layer.

FIG. 4 illustrates wiring breakage often occurring in the prior art TFTintegrated circuit. A TFT region 401 and a gate wiring 402 are formedover a substrate. An interlayer insulator 403 is formed on these regionand wiring. If the edges of the gate wiring 402 are sharp, theinterlayer insulator 403 cannot fully cover the gate wiring. Under thiscondition, if the second wiring, 404 and 405, is formed, it is likelythat the second layer breaks, as shown, at portions 406.

In order to prevent such wiring breakage, it is necessary to increasethe thickness of the second wiring. For example, it has been desired toincrease the thickness of the gate wiring about twofold. However, thismeans that the unevenness on the integrated circuit is increasedfurther. If a further wiring is required to be deposited, breakage dueto the thickness of the second wiring must be taken into consideration.Where an integrated circuit whose unevenness should be suppressed as ina liquid crystal display, it is substantially impossible to address theproblem by increasing the thickness of the second wiring.

In an integrated circuit, if a wiring breakage occurs even at one edgeof a step, then the whole circuit is made useless. Therefore, it isimportant to reduce wiring breakages at steps.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method offabricating a semiconductor integrated circuit with minimum wiringbreakages at steps and thus with improved production yield.

It is another object of the invention to provide a semiconductorintegrated circuit in which wiring breakages at steps are reduced to aminimum.

In the present invention, after forming gate electrodes and gate wiring,a silicon nitride film is formed at least on their top surfaces,preferably even on their side surfaces, by plasma CVD or sputtering.Then, a substantially triangular regions (side walls) is formed out ofthe insulator on the side surfaces of the gate electrodes and of thegate wiring by anisotropic etching. Subsequently, an interlayerinsulator is deposited, followed by formation of a second wiring.Silicon nitride exhibits a small etch rate under conditions in whichsilicon oxide forming the side walls is etched by dry etching.Therefore, the silicon nitride can be used as an etching stopper.

In a first method embodying the present invention, a semiconductor layerin the form of islands is first formed. A coating becoming agate-insulating film is formed on the semiconductor layer. Then, gateelectrodes and gate wiring are formed. Thereafter, silicon nitride isdeposited as a film to a thickness of 100 to 2000 Å, preferably 200 to1000 Å, by plasma-assisted CVD. Other CVD processes or sputteringtechniques can also be employed. Thus, the first step of the inventivemethod is completed.

Then, a coating of an insulator is formed on the silicon nitride. Inthis stage of formation of the coating, the coverage is important.Preferably, the thickness of the coating is one-third to 2 times theheight of the gate electrodes and the gate wiring. For this purpose,plasma-assisted CVD, LPCVD, atmospheric pressure CVD, and other CVDprocesses are preferably used. The insulator layer formed in this way ispreferentially etched in a direction substantially vertical to thesubstrate by anisotropic etching. The etching terminates at the surfaceof the silicon nitride. The underlying gate electrodes and gate wiringare prevented from being etched.

As a result, substantially triangular regions of an insulator, or sidewalls, are left on the side surfaces of the gate electrodes and gatewiring, because the coating of the insulator is intrinsically thick onsteps such as on the side surfaces of the gate electrodes and gatewiring. Thus, the second step of the inventive method is completed.

Then, an interlayer insulator is deposited. Contact holes are formed inone or both of source and drain regions of each TFT. The second wiringis formed, thus completing the third step of the inventive method.

Immediately after the side walls are formed in the second step, the filmof silicon nitride can be etched by dry etching. Preferably, thisetching step is performed while monitoring it with an endpoint monitoror other instrument. The etching of the film of silicon nitride can becontrolled well with the monitor. The thickness of the etched siliconnitride film is 100 to 2000 Å. Therefore, even if overetching occurs,the depth is much smaller than the thickness of the gate electrodes andgate-insulating film. Hence, the gate electrodes and gate-insulatingfilm are little affected thereby.

This method is effective where the gate-insulating film and theinterlayer insulator are made from the same material different fromsilicon nitride. That is, if the interlayer insulator layer is formedafter etching the silicon nitride film, the etching can be completed inone operation when the contact holes are formed.

Dopants are implanted to form the source and drain regions of each TFT.This implantation step can be varied variously. For example, where onlyN-channel TFTs are formed on a substrate, an N-type impurity may beintroduced into the semiconductor layer at a relatively highconcentration by self-alignment techniques, using the gate electrodes asa mask. This step is carried out between the first and second stepsdescribed above.

Similarly, where N-channel TFTs are formed, if they have the lightlydoped drain (LDD) structure, an impurity is introduced into thesemiconductor layer at a relatively low concentration. This step iseffected between the first and second steps described above; Then, anN-type impurity is introduced into the semiconductor layer at a higherconcentrations by self-alignment techniques, using the gate electrodesand the side walls as a mask. This step is performed between the secondand third steps described above. In this case, the width of the lightlydoped drains is approximate to the width of the side walls. Where onlyP-channel TFTs are formed on a substrate, similar steps may be carriedout.

Where offset TFTs are fabricated, an impurity is introduced into thesemiconductor layer at a high concentration, using the gate electrodesand side walls as a mask, by self-alignment techniques. This step iscarried out between the second and third steps described above. In thiscase, the width of the offset structure is approximate to the width ofthe side walls. In the TFT of this construction, the width of thesubstantially intrinsic region becoming a channel formation region isapproximately equal to the sum of the width of the gate electrode andthe widths of both side walls.

A complementary MOS (CMOS) circuit having N-channel TFTs and P-channelTFTs can be fabricated similarly on a substrate. Where N-channel TFTsand P-channel TFTs are composed of ordinary TFTs, or where both kinds ofTFTs are composed of LDD TFTs, an N-type impurity and a P-type impurityare implanted similarly to the above-described method in which only onekind of TFTs, or N-channel or P-channel TFTs, is formed on a substrate.

For example, where N-channel TFTs which are required to takecountermeasures against hot carriers are made of the LDD type andP-channel TFTs which are not required to take such countermeasures areboth made of ordinary TFTs, the impurity implantation step is a slightlyspecial step. In this case, an N-type impurity is introduced into thesemiconductor layer at a relatively low concentration. This step iscarried out between the first and second steps described above. This isreferred to as the first impurity introduction. At this time, the N-typeimpurity may be added even into the semiconductor layer of the P-channelTFTs.

Then, masking the semiconductor layer of the N-channel TFTs, a P-typeimpurity is introduced only into the semi-conductor layer of theP-channel TFTs at a higher concentration. This is referred to as thesecond impurity introduction. Even if the N-type impurity exists in theP-channel TFTs as a result of the previous introduction of the N-typeimpurity, the P-type impurity is introduced at a higher concentration asa result of the second impurity introduction. As a result, thesemi-conductor is rendered P-type. Of course, the concentration of thesecond impurity is greater than that of the first impurity. Preferably,the concentration of the second impurity is one to three orders ofmagnitude greater than that of the first impurity.

Finally, in order to form source/drain regions of the N-channel TFTs, anN-type impurity is introduced at a relatively high concentration. Thisstep is carried out between the second and third steps described above.This is referred to as the third impurity introduction. In this case, inorder to prevent the N-type impurity from being introduced into theP-channel TFTs, they may or may not be masked. In the latter case, it isnecessary that the concentration of the introduced N-type impurity belower than that of the P-type impurity introduced by the second impurityintroduction. Preferably, the concentration of the introduced N-typeimpurity is one-tenth to two-thirds of the concentration of the P-typeimpurity introduced by the second impurity introduction. As a result,the N-type impurity is introduced even into the P-channel TFTs but at alower concentration than the P-type impurity previously introduced.Therefore, the P-channel TFTs are maintained as P-type.

In the present invention, the presence of the side walls improves thestep coverage at portions at which the gate wiring extends over theinterlayer insulator layer, thus reducing breakages of the secondwiring. Furthermore, the lightly doped structure or the offset structurecan be obtained by making use of the side walls described above.

In the present invention, the existence of the silicon nitride film isof importance. In the above-described second step, anisotropic etchingis done to form the side walls. However, on a dielectric surface, it isdifficult to control plasma. The substrate is inevitably etchednon-uniformly.

The etching depth is one-third to 2 times as large as the height of thegate electrodes and gate wiring. Therefore, the nonuniform etchingproduces great effects. If no silicon nitride film is formed on the topsurfaces of the gate electrodes, the gate electrodes and gate wiringwill be etched severely at some locations within the same substrateduring the etching of the side walls.

If any silicon nitride film exists during the etching of the side walls,the etching stops at this location, thus protecting the gate electrodesand gate wiring. If the silicon nitride film is removed later by dryetching, the etching depth is much smaller than the etching depth in theside walls. Consequently, even if the gate electrodes and gate wiringare overetched, no great effects are produced.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A)-1(F) are cross sectional views for showing the manufacturingmethod in accordance with Example 1 of the present invention;

FIGS. 2(A)-2(F) are cross sectional views for showing the manufacturingmethod in accordance with Example 2 of the present invention;

FIGS. 3(A)-3(E) are cross sectional views for showing the manufacturingmethod in accordance with Example 3 of the present invention;

FIG. 4 is a cross section of a TFT circuit, illustrating the prior artfabrication method;

FIGS. 5(A)-5(F) are cross sectional views for showing the manufacturingmethod in accordance with Example 4 of the present invention;

FIGS. 6(A)-6(F) are cross sectional views for showing the manufacturingmethod in accordance with Example 5 of the present invention;

FIGS. 7(A)-7(F) are cross sectional views for showing the manufacturingmethod in accordance with Example 6 of the present invention;

FIGS. 8(A)-8(G) are cross sectional views for showing the manufacturingmethod in accordance with Example 7 of the present invention;

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

The present example is illustrated in FIGS. 1(A)-1(F). First, siliconoxide was deposited as a buffer layer 102 on a substrate 101 made ofCorning 7059 glass. The substrate 101 measured 300 mm×400 mm or 100mm×100 mm. The thickness of the buffer layer 102 was 1000 to 5000 Å,e.g., 2000 Å. The oxide film was formed in an oxygen ambient bysputtering or by plasma-assisted CVD, using TEOS as a raw material. Thesilicon oxide film formed in this way could be annealed at 400 to 650°C.

Then, an amorphous silicon film was deposited to a thickness of 300 to5000 Å, preferably 400 to 1000 Å, e.g., 500 Å, by plasma-assisted CVD orLPCVD. The laminate was allowed to stand in a reducing ambient of 550 to600° C. for 8 to 24 hours, thus crystallizing the amorphous film. Atthis time, a trace amount of a metal element for promotingcrystallization such as nickel could be added. This step could also uselaser irradiation. The silicon film crystallized in this way was etchedto form island regions 103. Then, silicon oxide was deposited as agate-insulating film 104 on the laminate to a thickness of 700 to 1500Å, e.g., 1200 Å, by plasma-assisted CVD.

Thereafter, an aluminum film having a thickness of 1000 Å to 3 μm, e.g.,5000 Å, was formed by sputtering and etched to form a gate electrode 105and a gate wiring 106. If an appropriate amount of silicon, copper,scandium, or other material were contained in the aluminum film,generation of hillocks could be suppressed when a silicon nitride filmwas formed subsequently. For example, where scandium was contained, itsconcentration was 0.1 to 0.3% by weight (FIG. 1(A)).

Subsequently, a silicon nitride film 107 was formed to a thickness of100 to 2000 Å, preferably 200 to 1000 Å, e.g., 500 Å, by plasma-assistedCVD using a mixture gas of NH₃, SiH₄, and H₂. Other CVD processes orsputtering methods may also be used but it is desired that the gateelectrode be covered with good step coverage.

Thereafter, using the gate electrode as a mask, an impurity (in thisexample, phosphorus) was implanted into the silicon film 103 in the formof islands by self-aligned ion doping techniques. In this way, lightlydoped regions 108 were formed, as shown in FIG. 1(B). The dose was1×10¹³ to 5×10¹⁴ atoms/cm². The accelerating voltage was 10 to 90 kV.For example, the dose was 5×10¹³ atoms/cm². The accelerating voltage was80 kV (FIG. 1(B)).

A silicon oxide was deposited as a film 109 by plasma-assisted CVD,using TEOS and oxygen as raw materials or using monosilane and nitrousoxide as raw materials. The optimum value of the silicon oxide film 109varies, depending on the height of the gate electrode and gate wiring.For example, where the height of the gate electrode and gate wiringincluding the silicon nitride film is about 5000 Å as in the presentexample, the optimum value is preferably one-third to 2 times thisvalue, i.e., 2000 Å to 1.2 μm. In this example, the value was 6000 Å. Inthis film formation step, the film thickness of planar portions isrequired to be uniform. In addition, the step coverage must be good. Asa result, the thickness of the silicon oxide film at the side surfacesof the gate electrode and gate wiring is increased by the portionsindicated by the broken lines in FIG. 1(C) (FIG. 1(C)).

This silicon oxide film 109 was etched by anisotropic etching, using thewell-known RIE process. This etching terminated at the surface of thesilicon nitride film 107. Since the silicon nitride film was not readilyetched by anisotropic etching using the RIE process, the etching did notprogress into the gate-insulating film 104. By the steps described thusfar, substantially triangular region of an insulator or side walls 110and 111 were left on the side surfaces of the gate electrode and gatewiring (FIG. 1(D)).

Then, phosphorus ions were implanted again by ion doping. In this case,the dose is preferably one to three orders of magnitude higher than thedose used in the step shown in FIG. 1(B). In the present example, thedose was 40 times as high as the dose of the initially implantedphosphorus, i.e., 2×10¹⁵ atoms/cm². The accelerating voltage was 80 kV.As a result, source/drain regions 113 heavily doped with phosphoruscreated. Lightly doped regions 112 were left under the side walls (FIG.1(E)).

The laminate was irradiated with KrF excimer laser light having awavelength of 248 nm and a pulse width of 20 nsec to activate thedopant. The energy density of the laser light was 200 to 400 mJ/cm²,preferably 250 to 300 mJ/cm². In the present example, the gate electrodewas made of aluminum. Since the gate electrode was capped with thesilicon nitride film 107, the gate electrode was not affected by thelaser irradiation. Instead of the laser irradiation, rapid thermalannealing (RTA) or rapid thermal processing (RTP) can be utilized.

Finally, silicon oxide was deposited as an interlayer insulator layer114 over the whole surface to a thickness of 5000 Å by CVD. Contactholes were created in the source/drain regions of the TFT. The secondwiring, or aluminum wiring 115 and aluminum electrodes 116, was formed.The thickness of the aluminum wiring was approximate to the thickness ofthe gate electrode and wiring, i.e., 4000 to 6000 Å.

TFTs having N-channel LDDs were completed by the manufacturing stepsdescribed thus far. In order to activate the doped regions, hydrogenannealing may be carried out at 200 to 400° C. The presence of the sidewalls 111 make milder the steps over which the second wiring 116 areextended. Therefore, little breakages were observed although thethickness of the second wiring was substantially the same as thethickness of the gate electrode and wiring (FIG. 1(F)).

When the thickness of the gate electrode/wiring is x (Å) and thethickness of the second wiring is y (Å), the following relation shouldbe satisfied in order to prevent wire disconnection.y≧x−1000(Å)

As the thickness y is reduced, more desirable results are obtained. Theinventors of the present invention have found that in the case of acircuit required to have a less uneven surface as in the active matrixcircuit of a liquid crystal display, if the relations given byx−1000(Å)≦y≦x+1000(Å)are met, then satisfactory results arise.

Example 2

FIGS. 2(A)-2(F) illustrate the second example of the present invention.The present example pertains to a monolithic active matrix circuithaving an active matrix circuit and a driver circuit for driving theactive matrix circuit. Both the active matrix circuit and the drivercircuit are formed on the same substrate. In the present example,P-channel TFTs are used for the switching devices of the active matrixcircuit. The driver circuit is a complementary circuit consisting ofN-channel and P-channel TFTs. Shown at the left sides of FIGS. 2(A)-2(F)are cross-sectional views of the N-channel TFT used in the drivercircuit, illustrating the process sequence. Shown at the right sides ofFIGS. 2(A)-2(F) are cross-sectional views of the P-channel TFT used inthe driver circuit or in the active matrix circuit, illustrating theprocess sequence. P-channel TFTs are used as the switching circuits ofthe active matrix circuit, because the leakage current (also called asOFF current) is small.

First, an insulating oxide was deposited as a buffer layer 202 on asubstrate 201 made of Corning 7059 glass, in the same way as inExample 1. A semiconductor region in the form of islands was formed onthe buffer layer 202. A silicon oxide film 203 acting as a gate oxidefilm was formed. Gate electrodes 204 and 205 were formed out of analuminum film having a thickness of 5000 Å. Then, in the same way as inExample 1, a silicon oxide film 206 having a thickness of 100 to 2000 Å,e.g., 1000 Å, was formed. Using the gate electrode as a mask, phosphorusions were implanted by ion doping. As a result, lightly doped N-typeregions 207 and 208 were formed. The dose was 1×10¹³ atoms/cm².

The laminate was irradiated with KrF excimer laser light having awavelength of 248 nm and a pulse width of 20 nsec to activate theimplanted dopant. The energy density of the laser light was 200 to 400mJ/cm², preferably 250 to 300 mJ/cm² (FIG. 2(A)).

Then, the N-channel TFT regions were masked with photoresist 209. Underthis condition, boron ions were implanted at a high dose of 5×10¹⁵atoms/cm² by ion doping. The accelerating voltage was 65 kV. As aresult, the region 208 which was lightly doped N-type by the previousimplant of phosphorus was changed into a strong P-type. Thus, a P-typedoped region 210 was formed. The laminate was then irradiated with laserlight to activate the dopant (FIG. 2(B)).

After removing the mask 209 of the photoresist, a silicon oxide film 211having a thickness of 4000 to 8000 Å was formed by plasma-assisted CVD(FIG. 2(C)).

Side walls 212 and 213 of silicon oxide were formed on the side surfacesof the gate electrode by anisotropic etching, in the same way as inExample 1 (FIG. 2(D)).

Then, phosphorus ions were implanted by ion doping. In this case, thedose was 1 to 3 orders of magnitude greater than the dose used in thestep illustrated in FIG. 2(A). Preferably, the dose is one-tenth totwo-thirds of the dose used in the step illustrated in FIG. 2(B). In thepresent example, the dose was 200 times as high as the dose of thephosphorus ions first implanted, i.e., 2×10¹⁵ atoms/cm². This is 40% ofthe dose of the boron used in the step illustrated in FIG. 2(B). Theaccelerating voltage was 80 kV. As a result, source/drain regions 214heavily doped with phosphorus were formed. Lightly doped drain (LDD)regions 215 were formed under the side walls.

The laminate was irradiated with KrF excimer laser light having awavelength of 248 nm and a pulse width of 20 sec to activate theimplanted dopant. The energy density of the laser light was 200 to 400mJ/cm², preferably 250 to 300 mJ/cm².

The P-channel TFT (the right side in the figure) was doped withphosphorus and maintained in P-type, because the concentration of thepreviously doped boron was 2.5 times as high as the concentration of thephosphorus. Apparently, the P-type region of the P-channel TFT had twokinds of regions, i.e., regions 217 under the side walls and outerregions 216 on the opposite side of the channel formation regions.However, these two kinds of regions made no great difference inelectrical characteristics (FIG. 2(E)).

Finally, as shown in FIG. 2(F), silicon oxide was deposited as aninterlayer insulator layer 218 to a thickness of 3000 Å over the wholesurface, as shown in FIG. 2(F). Contact holes were created in thesource/drain regions of the TFT. Aluminum wiring/electrodes 219, 220,221, 222 were created. A semiconductor integrated circuit in which theN-channel TFT was of the lightly doped drain (LDD) structure wascompleted.

The interlayer insulator layer was not very thick at the portion atwhich the second wiring went over the gate wiring. However, almost nowiring breakages were observed in the same way as in Example 1.

In the present example, the N-channel TFT assumed the LDD structure toprevent hot carriers from deteriorating the performance of the device.However, the LDD region was a parasitic resistor inserted in series withthe source/drain regions and so the operating speed dropped.Accordingly, in the case of the P-channel TFT which has a small mobilityand is less susceptible to deterioration due to hot carriers, it isdesired that no LDD exist as in the present example.

In the present example, the dopant was activated by laser irradiationwhenever a doping step was carried out. Alternatively, all activationsteps may be simultaneously performed immediately before all the dopingsteps were ended and the interlayer insulator layer was formed.

Example 3

The present example is illustrated in FIGS. 3(A)-3(E). The presentexample is an example of fabrication of a TFT, using formation of offsetregions employing side walls.

First, silicon oxide was formed as a buffer layer 302 on a substrate 301to a thickness of 1000 to 5000 Å, e.g., 2000 Å, in the same way as inExample 1. The silicon oxide film could be annealed at 400 to 650° C. Aregion 303 in the form of islands was formed by the method described inExample 1. A silicon oxide film 304 having a thickness of 700 to 1500 Å,e.g., 1200 Å, was formed by plasma-assisted CVD.

Then, a phosphorus-doped polycrystalline silicon film having a thicknessof 1000 Å to 3 μm, e.g., 5000 Å, was formed by LPCVD. This was etched toform a gate electrode 305 and a gate wiring 306 (FIG. 3(A)).

Thereafter, a silicon nitride film 307 was formed to a thickness of 100to 2000 Å, preferably 200 to 1000 Å, within a mixture gas of phosphine(NH₃), monosilane (SiH₄), and hydrogen (H₂) by plasma-assisted CVD. Thissilicon nitride film may also be formed by sputtering or other method.

A silicon oxide film 308 was deposited by plasma-assisted CVD, usingTEOS and oxygen as gaseous raw materials or using monosilane and nitrousoxide as gaseous raw materials. The optimum thickness of the siliconoxide film 110 varies, depending on the height of the gate electrode andgate wiring. For example, where the thickness of the gate electrode andgate wiring including the thickness of the silicon nitride film is about6000 Å as in the present example, the optimum thickness of the layer 308is preferably one-third to 2 times this value, i.e., 2000 Å to 1.2 μm.In this example, the thickness was 6000 Å. In this film formation step,the film thickness of planar portions is required to be uniform. Inaddition, the step coverage must be good (FIG. 3(B)).

This silicon oxide film 308 was etched by anisotropic etching, using thewell-known RIE process. This etching terminated at the surface of thesilicon nitride film 307. Since the silicon nitride film was not readilyetched by anisotropic etching using the RIE process, the etching did notprogress into the gate-insulating film 304. By the manufacturing stepsdescribed thus far, side walls 309 and 310, were left on the sidesurfaces of the gate electrode and gate wiring (FIG. 3(C)).

Then, phosphorus ions were introduced by ion doping techniques. The dosewas 1×10¹⁴ to 5×10¹⁷ atoms/cm². The accelerating voltage was 10 to 90kV. For example, the dose was 2×10¹⁵ atoms/cm². The accelerating voltagewas 80 kV. As a result, source/drain regions 311 doped with phosphoruswere formed. Phosphorus was not introduced into regions located underthe side walls. In this way, offset regions were created (FIG. 3(D)).

The laminate was irradiated with KrF excimer laser light having awavelength of 248 nm and a pulse width of 20 nsec to activate theintroduced dopant. The energy density of the laser light was 200 to 400mJ/cm², preferably 250 to 300 mJ/cm². Instead of the laser irradiation,thermal annealing may be effected.

Finally, a silicon oxide film was deposited as an interlayer insulatorlayer 312 to a thickness of 5000 Å over the whole surface by CVD.Contact holes were created in the source/drain regions of the TFT. Thesecond aluminum layer forming aluminum wiring 313 and 314 was formed.The thickness of the aluminum wiring was approximate to that of the gateelectrode/wiring, i.e., 4000 to 6000 Å.

A TFT having an N-channel offset region was completed by themanufacturing steps described thus far. In order to activate the dopedregions, the laminate may be subsequently subjected to hydrogenannealing conducted at 200 to 400° C. The presence of the side wall 310makes milder the step at which the second wiring 314 goes over the gatewiring 306. Therefore, almost no wiring breakages were observedirrespective of the fact that the thickness of the wiring of the secondlayer is approximate to that of the gate electrode/wiring (FIG. 3(D)).

Example 4

The present example is illustrated in FIGS. 5(A)-5(F). In the presentexample, a TFT having an N-channel offset region and a TFT having anN-channel LDD region are formed on the same substrate.

First, in the same way as in Example 1, an oxide film 502 acting as abuffer layer, a silicon semiconductor region in the form of islands, anda silicon oxide film 503 serving as a gate oxide film were formed on asubstrate 501. Gate electrodes 504 and 505 were formed out of analuminum film having a thickness of 5000 Å. Then, a silicon nitride film506 was formed to a thickness of 100 to 2000 Å, e.g., 1000 Å, in thesame way as in Example 1 (FIG. 5(A)).

Thereafter, the offset TFT regions were masked with photoresist 507.Under this condition, phosphorus ions were implanted into the TFT havingthe LDD region by ion doping, using the gate electrode as a mask. Thus,lightly doped N-type regions 508 were created. The dose was 1×10¹³atoms/cm², for example.

The laminate was irradiated with KrF excimer laser light having awavelength of 248 nm and a pulse width of 20 nsec to activate theintroduced dopant. The energy density of the laser light was 200 to 400mJ/cm², preferably 250 to 300 mJ/cm² (FIG. 5(B)).

After removing the mask 507 of the photoresist, a silicon oxide film 509having a thickness of 4000 to 8000 Å, e.g., 6000 Å, was formed byplasma-assisted CVD (FIG. 5(C)).

In the same way as in Example 1, the silicon oxide film 509 was etchedby anisotropic etching, in the same way as in Example 1. Side walls 510and 511 of silicon oxide were formed on the side surfaces of the gateelectrode (FIG. 5(D)).

Then, phosphorus ions were implanted again by ion doping. In this case,the dose is preferably one to three orders of magnitude higher than thedose used in the step shown in FIG. 5(B). In the present example, thedose was 200 times as high as the dose of the initially implantedphosphorus, i.e., 2×10¹⁵ atoms/cm². The accelerating voltage was 80 kV.As a result, source/drain regions 512 and 513 heavily doped withphosphorus were created. In the step illustrated in FIG. 5(B), an offsetregion was left under the side wall of the masked TFT. A lightly dopedregion 514 was left under the side wall of the TFT lightly doped withphosphorus.

Then, the laminate was irradiated with KrF excimer laser light having awavelength of 248 nm and a pulse width of 20 nsec to activate theintroduced dopant. The energy density of the laser light was 200 to 400mJ/cm², preferably 250 to 300 mJ/cm² (FIG. 5(E)).

Finally, as shown in FIG. 5(F), silicon oxide was deposited as aninterlayer insulator layer 515 to a thickness of 3000 Å over the wholesurface. Contact holes were created in the source/drain regions of theTFT. Aluminum wiring and electrodes 516, 517, 518, and 519 were created.A semiconductor integrated circuit in which the N-channel TFT was thelightly doped drain (LDD) structure was completed.

The interlayer insulator layer was not very thick at the portion (notshown) at which the second wiring went over the gate wiring. However,almost no wiring breakages were observed in the same way as in Example1.

In the present example, the dopant was activated by laser irradiationwhenever an implantation step was carried out. Alternatively, allactivation steps may be simultaneously performed immediately after allthe implantation steps were ended but before the interlayer insulatorlayer was formed.

In the description made in connection with FIGS. 5(A)-5(F), only theN-channel TFT was described. A CMOS circuit may be constructed byforming both an N-channel TFT and a P-channel TFT on the same substrate,in the same way as in Example 2. For example, in a monolithic activematrix circuit comprising a substrate on which a peripheral circuit andan active matrix circuit are both formed, a CMOS circuit using an LDDN-channel TFT having a high operating speed and an ordinary NMOS TFT isused as the peripheral circuit. N- or P-channel offset TFTs are used asthe active matrix circuit which is required to exhibit low leakagecurrent. Especially, the P-channel offset TFT is effective in reducingthe leakage current. Of course, both N- and P-channel types can becomposed of LDD TFTs.

Example 5

The present example is illustrated in FIGS. 6(A)-6(F). First, siliconoxide was formed as a buffer layer 602 on a substrate 601 to a thicknessof 1000 to 5000 Å, e.g., 2000 Å, in the same way as in Example 1. Then,in the same way as in Example 1, a silicon region in the form of islandswas formed to a thickness of 500 Å. Silicon oxide was deposited as agate-insulating film 603 on the laminate to a thickness of 700 to 1500Å, e.g., 1200 Å, by plasma-assisted CVD.

Then, a gate electrode 604 and a gate wiring 605 were formed out of analuminum film having a thickness of 5000 Å. Furthermore, a siliconnitride film 606 was deposited to a thickness of 100 to 2000 Å,preferably 200 to 1000 Å, e.g., 500 Å, by plasma-assisted CVD.

Thereafter, using the gate electrode as a mask, an impurity (in thisexample, phosphorus) was implanted into the silicon film in the form ofislands by self-aligned implantation techniques. In this way, lightlydoped regions 607 were formed, as shown in FIG. 6(A). The dose was1×10¹³ to 5×10¹⁴ atoms/cm². The accelerating voltage was 10 to 90 kV.For example, the dose was 5×10¹³ atoms/cm². The accelerating voltage was80 kV (FIG. 6(A)).

A silicon oxide film 608 was deposited by plasma-assisted CVD. Thethickness was 6000 Å. In this film formation step, the film thickness ofplanar portions is required to be uniform. In addition, the stepcoverage must be good (FIG. 6(B)).

Then, an anisotropic dry etching step was conducted, using CHF₃ to etchthe silicon oxide film 608. At this time, the etching can be performeduntil it goes to the silicon nitride film 606. Preferably, as shown inFIG. 6(C), the etching is stopped immediately before the etching reachesthe silicon nitride film 606 so that a slight amount of the siliconoxide film 608 is left behind. Substantially triangular regions ofinsulator, or side walls 609 and 610, were left on the side surfaces ofthe gate electrode/wiring by the manufacturing steps described thus far(FIG. 6(C)).

Subsequently, a dry etching step was carried out, using CH₄ and O₂ toetch away the slight amount of silicon oxide film left on the siliconnitride film, as well as the silicon nitride film. Since this etchingstep can be monitored with an endpoint monitor (plasma monitor), thegate electrode and the gate-insulating film are prevented from beingoveretched (FIG. 6(D)).

Then, phosphorus ions were implanted again by ion doping. In this case,the dose is preferably one to three orders of magnitude higher than thedose used in the step shown in FIG. 6(A). In the present example, thedose was 40 times as high as the dose of the initially implantedphosphorus, i.e., 2×10¹⁵ atoms/cm². The accelerating voltage was 80 kV.As a result, source/drain regions 611 heavily doped with phosphorus werecreated. Lightly doped regions 612 were left under the side walls.

Then, the laminate was irradiated with KrF excimer laser light having awavelength of 248 nm and a pulse width of 20 nsec to activate theintroduced dopant. The energy density of the laser light was 200 to 400mJ/cm², preferably 250 to 300 mJ/cm² (FIG. 6(E)).

Finally, a silicon oxide film was deposited as an interlayer insulatorlayer 613 to a thickness of 5000 Å over the whole surface by CVD.Contact holes were created in the source/drain regions of the TFT. Thesecond aluminum layer forming aluminum wiring 614 and 615 was formed.The thickness of the aluminum wiring was approximate to that of the gateelectrode/wiring, i.e., 4000 to 6000 Å.

A TFT having an N-channel offset region was completed by themanufacturing steps described thus far. In order to activate the dopedregions, the laminate may be subsequently subjected to hydrogenannealing conducted at 200 to 400° C. In the same way as in Example 1,the presence of the side wall 610 makes milder the step at which thesecond wiring 613 goes over the gate wiring 605. Therefore, almost nowiring breakages were observed irrespective of the fact that thethickness of the wiring of the second layer is approximate to that ofthe gate electrode/wiring (FIG. 6(F)).

In the present example, the silicon nitride film 606 was etched, and thegate-insulating film 603 was exposed. This enabled contact holes to beformed in one operation, i.e., wet etching process. As can be seen fromFIG. 6(E), as a result of the etching of the silicon nitride film, thesilicon nitride film was left only between the gate electrode 604 or 605and each of the side walls 609 and 610 or between the gate-insulatingfilm 603 and the side walls 609 and 610.

Example 6

The present example is illustrated in FIGS. 7(A)-7(F). In the presentexample, in the same way as in Example 2, an LDD N-channel TFT and anordinary P-channel TFT are formed on the same substrate. The left sidesof FIGS. 7(A)-7(F) are cross sections of an N-channel TFT, illustratingthe process sequence for fabricating the TFT. The right sides of FIGS.7(A)-7(F) are cross sections of a P-channel TFT, illustrating theprocess sequence for fabricating the TFT. First, an oxide film 702acting as a buffer layer, a silicon semiconductor layer in the form ofislands, and a silicon oxide film 703 acting as a gate oxide film wereformed on a substrate 701 made of Corning 7059 glass. Then, gateelectrodes 704 and 705 were formed out of an aluminum film having athickness of 5000 Å.

Then, using the gate electrode 704 as a mask, the gate oxide film in theportion of the N-channel TFT was selectively removed to expose thesemiconductor layer. Subsequently, a silicon nitride film 706 having athickness of 100 to 2000 Å, preferably 200 to 1000 Å, e.g., 400 Å, wasformed by plasma-assisted CVD.

Using the gate electrode as a mask, phosphorus ions were implanted byion doping to form lightly doped N-type regions 707. The dose was 1×10¹³atoms/cm². The accelerating voltage was 20 keV. During this dopingprocess, the accelerating voltage was low. Therefore, phosphorus ionswere not implanted into the islands 708 of the P-channel TFT coated withthe gate oxide film 703 (FIG. 7(A)).

Then, the N-channel TFT regions was masked with photoresist 709. Underthis condition, boron ions were implanted at a high concentration by iondoping. The dose was 5×10¹⁴ atoms/cm². The accelerating voltage was 65kV. As a result, P-type doped regions 710 were formed in the islands 708(FIG. 7(B)).

In the present example, after the whole surface was lightly doped withphosphorus, the surface was selectively heavily doped with boron. Thesequence in which these two steps are carried out may be reversed.

After removing the photoresist mask 709, a silicon oxide film 711 havinga thickness of 4000 to 8000 Å was formed by plasma-assisted CVD (FIG.7(C)).

Then, side walls 712 and 713 of silicon oxide were formed on the sidesurfaces of the gate electrode by anisotropic etching, similarly toExample 2 (FIG. 7(D)).

Then, phosphorus ions were introduced again by ion doping. In this case,the dose is preferably one to three orders of magnitude higher than thedose used in the step shown in FIG. 7(A). In the present example, thedose was 200 times as high as the dose of the initially implantedphosphorus, i.e., 2×10¹⁵ atoms/cm². The accelerating voltage was 20 kV.As a result, source/drain regions 714 heavily doped with phosphorus werecreated. Lightly doped regions 715 were left under the side walls.

On the other hand, the P-channel region was not doped with phosphorusions because of the presence of the gate oxide film. In Example 2, theP-channel TFT was doped heavily with both phosphorus ions and boron ionsand so the limitations are imposed on the their doses. In the presentexample, no limitations are placed on the doses. However, with respectto the accelerating voltage, it must be set low for phosphorus ions andset high for boron ions (FIG. 7(E)).

Then, the laminate was irradiated with KrF excimer laser light having awavelength of 248 nm and a pulse width of 20 nsec to activate theintroduced dopant. The energy density of the laser light was 200 to 400mJ/cm², preferably 250 to 300 mJ/cm².

Finally, as shown in FIG. 7(F), silicon oxide was deposited as aninterlayer insulator layer 716 to a thickness of 5000 Å over the wholesurface by CVD. Contact holes were created in the source/drain regionsof the TFTs. Aluminum electrodes and aluminum wiring 717, 718, 119, and720 were created. A semiconductor integrated circuit consisting of theN-channel TFT made of the lightly doped drain (LDD) structure wascompleted by the manufacturing steps described thus far.

Compared with Example 2, the present example further needs aphotolithography step and an etching step to remove the gate oxide filmfrom the N-channel TFT portion. However, substantially no N-typeimpurity was implanted into the P-channel TFT and, therefore, the dosesof the N-type and P-type impurities can be relatively arbitrarilychanged.

The phosphorus ions were implanted into the portions close to thesurface of the gate oxide film 703 of the P-channel TFT. The phosphorusions will form phosphosilicate glass in the later laser irradiationstep. This is effective in preventing movable ions such as sodium ionsfrom entering.

Example 7

The present example is illustrated in FIGS. 8(A)-8(G). The presentexample relates to a method of fabrication of an active matrix liquidcrystal display. This method is now described by referring to FIGS.8(A)-8(G). Two TFTs located at the left sides of FIGS. 8(A)-8(G) arecomposed of an LDD N-channel TFT and an ordinary P-channel TFT,respectively. These TFTs are logic circuits used in a peripheral circuitor the like. The TFT shown at the right is a switching transistor usedin an active matrix array. The right TFT is an offset P-channel TFT.

First, an oxide film acting as a buffer layer, a silicon semiconductorregion in the form of islands, and a silicon oxide film 803 acting as agate oxide film were formed on a substrate made of Corning 7059 glass.The silicon semiconductor region in the form of islands is composed ofan island region 801 for a peripheral circuit and an island region 802for an active matrix circuit. Gate electrodes 804 and 805 for theperipheral circuit were formed out of an aluminum film having athickness of 5000 Å. Also, a gate electrode 806 for the active matrixcircuit was formed out of the aluminum film.

Then, using the gate electrodes 804 and 806 as a mask, the gate oxidefilm in the portions of the P-channel TFTs for the peripheral circuitand for the active matrix circuit was selectively removed to expose thesemiconductor layer. Subsequently, a silicon nitride film 808 having athickness of 100 to 2000 Å, preferably 200 to 1000 Å, e.g., 600 Å, wasformed by plasma-assisted CVD.

The active matrix circuit region was masked with photoresist 807. Usingthe gate electrode 804 as a mask, phosphorus ions were implanted by iondoping to form heavily doped P-type regions 809. The dose was 1×10¹⁵atoms/cm². The accelerating voltage was 20 keV. During this dopingprocess, the accelerating voltage was low. Therefore, phosphorus ionswere not implanted into the N-channel TFT regions coated with the gateoxide film 803 (FIG. 8(A)).

Then, phosphorus ions were implanted at a low concentration by iondoping. The dose was 1×10¹³ atoms/cm². The accelerating voltage was 80kV. As a result, lightly doped N-type regions 810 were created in theregions of the N-channel TFTs (FIG. 8(B)).

In the illustrated example, the ions were implanted after removing thephotoresist mask 807. The ion implantation may be made while leaving thephotoresist. Since the phosphorus ions are accelerated at a highvoltage, if ion implantation is done while leaving the photoresist, thenthe phosphorus ions are not implanted into the active matrix circuitregions. Therefore, ideal offset P-channel TFTs are obtained. However,as a result of the ion implantation, the photoresist is carbonized. Itmay be laborious to remove this carbonized photoresist.

Even if the photoresist is removed, the phosphorus concentrationexhibits peaks under the semiconductor region in the form of islands,because the accelerating voltage for phosphorus is high. However, it isnot assured that phosphorus ions are not implanted at all. Rather, atrace amount of phosphorus is introduced into the semiconductor region.Even if phosphorus is implanted in this way, the concentration is quitelow. Furthermore, a structure given by P+ (source), N−, I (channel)/N−,P+ (drain) is formed. This is best suited for a TFT for an active matrixcircuit that is required to reduce its leakage current.

Then, a silicon oxide film was deposited to a thickness of 4000 to 8000Å by plasma-assisted CVD. Side walls 811, 812, and 813 of silicon oxidewere formed on the side surfaces of the gate electrode (FIG. 8(C)).

Thereafter, boron ions were again implanted by ion doping. In this case,the dose is preferably approximate to the dose used in the stepillustrated in FIG. 8(A). In the present example, the dose was 1×10¹⁵atoms/cm². The accelerating voltage was 20 keV. Since the acceleratingvoltage was low, the boron ions were not implanted into the N-channelTFT regions on which the gate oxide film 803 existed. The boron ionswere chiefly implanted into the source/drain regions of the P-channelTFTs of the r peripheral circuit and of the active matrix circuit. As aresult, source/drain regions 814 of the TFT of the active matrix circuitwere created. In each of these TFTs, the gate electrode is remote fromthe source/drain regions, i.e., the offset structure (FIG. 8(D)).

Then, phosphorus ions were implanted. Preferably, the dose is one tothree orders of magnitude higher than the dose used in the stepillustrated in FIG. 8(B). In the present example, the dose was 50 timesas high as the dose of the phosphorus ions first implanted, i.e., 5×10¹⁴atoms/cm². The accelerating voltage was 80 kV. As a result, regions 815heavily doped with phosphorus were created. Lightly doped drain (LDD)regions 816 were formed under the side walls.

On the other hand, in the P-channel TFT regions, many of the phosphorusions were implanted into the buffer layer. The conductivity type was notaffected greatly (FIG. 8(E)).

After the ion implantation, the laminate was irradiated with KrF excimerlaser light having a wavelength of 248 nm and a pulse width of 20 nsecto activate the implanted dopant. The energy density of the laser lightwas 200 to 400 mJ/cm², preferably 250 to 300 mJ/cm².

Then, as shown in FIG. 8(F), silicon oxide was deposited as a firstinterlayer insulator layer 817 to a thickness of 5000 Å over the wholesurface by CVD. Contact holes were created in the source/drain regionsof the TFTs. Aluminum electrodes and aluminum wiring 818, 819, 820, and821 were created. Peripheral circuit regions were formed by themanufacturing steps described above.

Silicon oxide was deposited as a second interlayer insulator layer 822to a thickness of 3000 Å by CVD. This was etched, and contact holes werecreated. Pixel electrodes 823 were formed out of transparent conductivefilm in TFTs of an active matrix circuit. In this way, an active matrixliquid crystal display device was fabricated (FIG. 8(G)).

In the present invention, the thickness of the second wiring can be madeto approximate the thickness of the gate electrodes and gate wiring.More specifically, the thickness of the second wiring can be equal tothe thickness of the gate electrodes and gate wiring ±1000 Å. This iswell suited to an active matrix circuit for a liquid crystal displaywhose plates are required to be less uneven.

While the preferred embodiments of the invention have been described, itis to be understood that various modifications may be made by thoseordinary skilled in the art. For example, before forming a siliconnitride film on a gate electrode, it is possible to form an oxide layeron the gate electrode by anodic oxidation.

1. A semiconductor device comprising: a gate electrode of a transistorand a wiring which is an extension of the gate electrode formed over aninsulating surface; a side wall formed adjacent to a side surface of thegate electrode; a first insulating film interposed between the side walland the side surface of the gate electrode, and extending below the sidewall; and a second insulating film formed over and being in contact withthe gate electrode of the transistor, the side wall, the firstinsulating film and the wiring.
 2. A semiconductor device according toclaim 1, wherein each of the gate electrode and the wiring comprisesaluminum.
 3. A semiconductor device according to claim 1, wherein thefirst insulating film comprises a silicon nitride film.
 4. Asemiconductor device according to claim 1, wherein the second insulatingfilm comprises a silicon oxide film.
 5. A semiconductor device accordingto claim 1, wherein the side wall comprises a silicon oxide film.
 6. Asemiconductor device according to claim 1, wherein the transistor is athin film transistor.
 7. A semiconductor device according to claim 1,wherein the gate electrode of the transistor is not overlapped with alightly doped region of the transistor.
 8. A semiconductor deviceaccording to claim 1, wherein an etching rate of the first insulatingfilm is different from that of the side wall.
 9. A semiconductor deviceaccording to claim 1, wherein a thickness of the gate electrode of thetransistor and a thickness of the wiring are 4000 to 6000 Å.
 10. Asemiconductor device according to claim 1, wherein the wiring is a gatewiring.
 11. A semiconductor device comprising: a gate electrode of atransistor and a first wiring which is an extension of the gateelectrode formed over an insulating surface; a side wall formed adjacentto a side surface of the gate electrode; a first insulating filminterposed between the side wall and the side surface of the gateelectrode, and extending below the side wall; a second insulating filmformed over and being in contact with the gate electrode of thetransistor, the side wall, the first insulating film and the firstwiring; and a second wiring formed over the second insulating film andcrossing over the first wiring.
 12. A semiconductor device according toclaim 11, wherein each of the gate electrode, the first wiring and thesecond wiring comprises aluminum.
 13. A semiconductor device accordingto claim 11, wherein the first insulating film comprises a siliconnitride film.
 14. A semiconductor device according to claim 11, whereinthe second insulating film comprises a silicon oxide film.
 15. Asemiconductor device according to claim 11, wherein the side wallcomprises a silicon oxide film.
 16. A semiconductor device according toclaim 11, wherein the transistor is a thin film transistor.
 17. Asemiconductor device according to claim 11, wherein the gate electrodeof the transistor is not overlapped with a lightly doped region of thetransistor.
 18. A semiconductor device according to claim 11, wherein anetching rate of the first insulating film is different from that of theside wall.
 19. A semiconductor device according to claim 11, wherein athickness of the gate electrode of the transistor and a thickness of thefirst wiring are 4000 to 6000 Å.
 20. A semiconductor device according toclaim 11, wherein the first wiring is a gate wiring.